I would suggest the Smitty Harris tap code... inscribe the 26 letters of the English alphabet in a 5x5 matrix, doubling up C and K in the same cell...as they have the same sound....

-- 1 2 3 4 5
1- a b c d e

2- f g h i j

3- l m n o p

4- q r s t u

5- v w x y z

tap out in row column order 5,2 / 1,1 / 4,4 / 1,5 / 4,2
w a t e r

sorry, I'm not able to get the correct spacing... from the Hoa Lo prison, it is my understanding that the first row was sent without a row indicator, only the column, that is tap,tap,tap,tap,tap for "e" to speed up the message